But the EWE redox project deploys recyclable polymers mixed in salt water "like cocoa in milk" - and is dubbed brine-for-power or b4p.

The first €100-120 million commercial battery will involve use of two medium-sized caverns.

The project is to have 120MW capacity and electricity release capability of 750MWh per hour.

It is expected to have around 20,000 charging cycles meaning a lifetime of about 20 years and no power leakage, according to EWE.

"We need to carry out more tests and clarify several issues before we can use the storage principle developed by the University of Jena for underground caverns. However, I expect that we will have an operating cavern battery by about the end of 2023," says Ralf Riekenberg, head of the brine4power project.

The first pilot plant using two above-ground container-sized vessels for the electrolytes will be commissioned at Jemgo before the end of 2017 at an investment of €2 million.